Show that the angle sum of any quadrilateral is 27.

Elementary Geometry For College Students, 7e
7th Edition
ISBN:9781337614085
Author:Alexander, Daniel C.; Koeberlein, Geralyn M.
Publisher:Alexander, Daniel C.; Koeberlein, Geralyn M.
Chapter2: Parallel Lines
Section2.2: Indirect Proof
Problem 11E: Which of the following statements would you prove by the indirect method? a In triangle ABC, if...
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ometry
of N and M, not
s line equals M.
is unique.
el to L through
ed ASA ("angle
shown in Fig-
least one side
y between L
e point where
etermined by
But then an
ence Land
assumption
that there is
d M are in
ollow from
clid's par-
it, so the
, there is
om," af-
book in
it does
en need
ate in-
n such
mply
gle,
2.1 The parallel axiom
To prove this property, draw a line L through one vertex of the trian-
gle, parallel to the opposite side, as shown in Figure 2.3.
L
a
a
В
Deduce from
only for n = 3,4,6.
Y
23
Y
Figure 2.3: The angle sum of a triangle
Then the angle on the left beneath L is alternate to the angle a in the
triangle, so it is equal to a. Similarly, the angle on the right beneath Lis
equal to y. But then the straight angle л beneath Lequals a+B+y, the
angle sum of the triangle.
Exercises
The triangle is the most important polygon, because any polygon can be built
from triangles. For example, the angle sum of any quadrilateral (polygon with
four sides) can be worked out by cutting the quadrilateral into two triangles.
Show that the angle sum of any quadrilateral is 27.
A polygon P is called convex if the line segment between any two points in
P lies entirely in P. For these polygons, it is also easy to find the angle sum.
Explain why a convex n-gon can be cut into n - 2 triangles.
Use the dissection of the n-gon into triangles to show that the angle sum of
a convex n-gon is (n − 2).
to find the angle at each vertex of a regular n-gon (an
n-gon with equal sides and equal angles).
that copies of a regular n-gon can tile the plane
Transcribed Image Text:ometry of N and M, not s line equals M. is unique. el to L through ed ASA ("angle shown in Fig- least one side y between L e point where etermined by But then an ence Land assumption that there is d M are in ollow from clid's par- it, so the , there is om," af- book in it does en need ate in- n such mply gle, 2.1 The parallel axiom To prove this property, draw a line L through one vertex of the trian- gle, parallel to the opposite side, as shown in Figure 2.3. L a a В Deduce from only for n = 3,4,6. Y 23 Y Figure 2.3: The angle sum of a triangle Then the angle on the left beneath L is alternate to the angle a in the triangle, so it is equal to a. Similarly, the angle on the right beneath Lis equal to y. But then the straight angle л beneath Lequals a+B+y, the angle sum of the triangle. Exercises The triangle is the most important polygon, because any polygon can be built from triangles. For example, the angle sum of any quadrilateral (polygon with four sides) can be worked out by cutting the quadrilateral into two triangles. Show that the angle sum of any quadrilateral is 27. A polygon P is called convex if the line segment between any two points in P lies entirely in P. For these polygons, it is also easy to find the angle sum. Explain why a convex n-gon can be cut into n - 2 triangles. Use the dissection of the n-gon into triangles to show that the angle sum of a convex n-gon is (n − 2). to find the angle at each vertex of a regular n-gon (an n-gon with equal sides and equal angles). that copies of a regular n-gon can tile the plane
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